The bosses in Sonic Superstars. The bosses are so slow to do their cycles that you often have to wait 30+ seconds between opportunities to damage them. Combine this with one-hit deaths in later stages and a lack of checkpoints and you have an incredibly frustrating experience. Just look at the video above, a successful attempt at this boss takes almost 10 minutes and half of it is just waiting for a chance to attack. There are also points in this boss fight where you have to stand in a certain point on the screen with very little warning else you die instantly and have to redo another 5+ minutes of the same boring fight to get another chance.
I liked the stages and the game overall but the bosses are among the worst I've ever seen.